<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="../js/vue.js"></script>
</head>
<body>
  <div id="app">
      <p>{{km}}</p>
      <p>{{m}}</p>
  </div>
<br>
  <script>
    let app = new Vue({
      el:"#app",
      data:{
           km:0,
           m:0
       },
       methods:{
        kmtom:function(){
            this.m=this.km*1000;
        },
        mtokm:function(){
            this.km=this.m/1000;
        }
       },
       watch:{
        km:'kmtom',
        m:'mtokm'
       }
     })
   </script>
</body>
</html>